1. Field of the Invention
The present invention relates to improvements on a discharging or diselectrifying mechanism provided in an image forming apparatus and, more specifically, to an image forming apparatus, which is equipped with a mechanism for effectively erasing positive charge remaining on a photosensitive drum of the image forming apparatus and also for making possible smooth build-up of charging.
2. Description of the Related Art
An image forming apparatus as represented by a copying machine is generally constructed including a photosensitive drum (i.e. a latent image carrier), an electrostatic charging device or charger for imparting a surface electrical charge to the photosensitive drum to form a desired latent image thereon, exposure means for altering the surface potential on the photosensitive drum by exposure with a laser beam thereby to form the desired latent image, a developing apparatus equipped with a developing roller for holding and feeding a toner onto the photosensitive drum to visualize the latent image carried on the drum, a transfer device for transferring the visualized image onto a sheet of image recording paper, a cleaning device for cleaning the photosensitive drum surface by removing and collecting the residual toner remaining on the drum after the image has been transferred to the image recording paper sheet, and a diselectrifying or erase lamp for erasing the residual charge for the latent image remaining on the photosensitive drum surface after the image has been transferred to the image recording paper sheet.
In the image forming apparatus with such an arrangement, although the toner stuck on the photosensitive drum surface is transferred onto the image recording paper sheet via the transfer device, the toner is not necessarily transferred entirely, but part of the toner is attached as the residual toner on the drum surface in an image-shaped distribution. Such residual toner is removed by the cleaning device and recovered in a waste toner recovery receiver. In this manner, the photosensitive drum surface is cleaned and, thereafter, the entire photosensitive drum surface is subjected to light exposure by the discharging lap, thus erasing the charge existing on the drum surface for the next operation including the steps of charging and exposure.
For the discharging or diselectrifying step to erase the electrical charge on the photosensitive drum surface, various method are known, including discharging by radiating light from any light source against the photosensitive drum surface, AC discharging by corona charging the photosensitive drum surface, and discharging by application of AC voltage with a cleaning blade coated with a conductive material such as carbon black or aluminum powder.
Such conventional discharging methods have had various problems as follows. In the method using light radiation, during the step of printing image on a recording paper sheet, the portion on the photosensitive drum surface which is reverse-charged by direct transfer charging, without via an image recording paper sheet in contact with the photosensitive drum, may fail to be discharged by light radiation, thus fogging appearing on such portion on the photosensitive drum surface. In a high-speed or small-size printer in which the period of time until the photosensitive drum is charged subsequent to its discharging is very short, a charge potential may be varied with variation in the discharging light amount unless the carrier movement speed of the photosensitive drum is fast enough.
In the AC discharging method using corona charging, it is necessary to provide an additional space for installation of a corona charger. This makes it difficult to construct a photosensitive drum having a smaller diameter and hence infeasible to apply to a small-size printer. Furthermore, the method of using the conductive cleaning blade is disadvantageous in that the blade surface may be partially peeled off or dropped out by friction of the blade edge coated with conductive material with the photosensitive drum. Especially, in a developing system using toner of small-diameter particles, poor cleaning may occur disadvantageously.
In view of a limited installation space for a positive charge erasing mechanism, it is difficult to install it in a small-size printer having a photosensitive drum with a diameter of about 30 mm, and a conductive blade is used to aid in diselectrifying the positive charge. Since such a blade is coated on it""s surface with conductive material, however, repeated passage of image recording paper sheets will peel off or nick at the edge surface of the blade, thereby inviting a decrease in discharging capability and/or poor cleaning, which in turn causes deteriorated image formation.
A discharging mechanism is disclosed in Japanese Patent Application KOKAI Publication No. 3-127086. This mechanism makes it possible to form quality images merely by using a conductive brush which performs the function of cleaning and discharging, dispensing with a cleaning device. Since the conductive blade does cleaning, however, the cleaning effect becomes inevitably poor with an increase in the number of prints. Thus, the diselectrification efficiency is affected by an increasing number of prints and the image quality is decreased, accordingly. Thus, the problem remains unsolved.
The present invention has been made in view of the above-described problems and, therefore, it is an object of the present invention to provide an image forming apparatus which is capable of producing high-quality images by diselectrifying positive charge with high efficiency, in particular, on a small-diameter photosensitive drum and hence permitting rapid build-up of charging.
For achieving this object, the invention provides an image forming apparatus comprising: a photosensitive member for forming thereon a latent image; a developing apparatus for visualizing said latent image on said photosensitive member with a toner having an electrical resistance; a transfer device for transferring said visualized image to an image recording medium; and bias means for applying a bias voltage of a predetermined magnitude to residual toner remaining on the surface of said photosensitive member after the transfer to said image recording medium by said transfer device, whereby at least part of surface potential of said photosensitive member is removed.
With such an arrangement, since the surface potential of the photosensitive drum is diselectrified by application of a bias voltage to the residual toner remaining on the surface of the photosensitive drum, the surface of the photosensitive drum, which is in contact with the residual toner, can be discharged efficiently. Additionally, the discharging mechanism can not only be simplified, but also the period of time until the photosensitive drum is charged after its discharging can be shortened, with the result that the image forming apparatus can be made compact suitable for a small-size image forming unit.
The image forming apparatus of the present invention is characterized in that it further comprises a non-conductive cleaning blade for removing the residual toner adhering on the surface of the photosensitive member after the transfer of the image onto the image-transferred material, and a conductive polyethylene terephalate (i.e., Mylar(copyright)) fixedly attached to the tip end of the cleaning blade, the conductive polyethylene terephalate being arranged so as to be applied with a voltage and operable to charge the residual toner remaining on the surface of the photosensitive member by charge injection from the voltage applied to the conductive polyethylene terephalate itself. Discharging can be accomplished more effectively by providing means such as transport paddle for making part of the residual toner to be accumulated uniformly over the surface of the photosensitive member.
With such a structure, the waste toner (residual toner) removed by cleaning with the cleaning blade is accumulated uniformly on the photosensitive member surface and a voltage is applied by charge injection from the conductive polyethylene terephalate which is then charged by a bias voltage. By so doing, the residual charge existing on the photosensitive member can be diselectrified by the electrically charged residual toner and the charge build-up performance in the next charging step can be accelerated. Moreover, the use of the conductive polyethylene terephalate permits utilization of a conventional non-conductive cleaning blade which is made of an elastic material such as rubber, thereby making it possible to maintain satisfactory cleaning capability.
The image forming apparatus according to the invention is further characterized in that the conductive polyethylene terephalate is applied with a bias voltage which is formed by imposing DC voltage of xe2x88x920.7 kV or more on AC voltage of 1.2 kV or more with a frequency of 300 Hz or greater.
In an image forming unit constructed according to the present invention and using a low-resistant toner such as magnetic toner, a non-conductive cleaning blade equipped with the conductive polyethylene terephalate is located above the photosensitive member. With such an arrangement, applying to the conductive polyethylene terephalate a bias voltage which is formed by imposing DC voltage of xe2x88x920.7 kV or greater on AC voltage of 1.2 kV or more with a frequency of 300 Hz or more thereby to electrically charge the residual toner by charge injection makes it possible to diselectrify with maximum efficiency the residual charge present on the surface of the photosensitive drum which is then in contact with the residual toner.
The image forming apparatus according to the invention is further characterized in that the cleaning blade is made of an elastic material.
By providing a cleaning blade which is made of a non-conductive elastic material such as rubber and hence flexible, the blade can be used in various shapes or forms suitable for maintenance of good cleaning performance. Thus, cleaning of the surface of a cylindrical shaped photosensitive drum can be done effectively.
The image forming apparatus according to the invention is still further characterized in that the photosensitive member includes a photosensitive drum with a diameter of 30 mm or less for use in a small-size image forming unit.
Since the cleaning and discharging can be effected merely by attaching a conductive polyethylene terephalate to the tip end of a cleaning blade, the apparatus can be made compact in size and hence applicable advantageously to a small-size unit using a photosensitive drum with a diameter of 30 mm or less.
The image forming apparatus according to the invention is also characterized in that the cleaning blade and the conductive polyethylene terephalate cooperate to form a cleaning device and the waste toner accumulated in the cleaning device is collected by a rotatable paddle.
The provision of the rotatable paddle (i.e., transport paddle) adjacent to the cleaning device which is located above the photosensitive drum and constructed to include a cleaning blade and a conductive polyethylene terephalate permits efficient collection of residual waste toner into a toner collection box.
Furthermore, the image forming apparatus according to the invention is characterized in that the photosensitive member comprises an organic photo conductor which is negatively charged for image formation, the conductive polyethylene terephalate being operable to erase residual positive charge remaining on the surface of such organic photo conductor by negatively charging the toner accumulated on the surface thereof after the transferring step.
It is significant to note that there is an increasing demand for an organic photo conductor (OPC) for application to an image forming apparatus for its advantages in terms of cost, safety and wavelength selectivity and, therefore, most of today""s new products of image forming apparatus employ OPC. Injection of negative charge to the residual toner by way of the conductive polyethylene terephalate can erase easily the residual positive charge on the OPC surface.
Still furthermore, the image forming apparatus according to the invention is characterized in that erasing the positive charge on the surface of the organic photo conductor assists in improving the build-up time in charging the organic photo conductor in the subsequent cycle of image forming operation.
According to the image forming apparatus of the invention wherein the positive residual charge on the surface of the organic photo conductor can be erased by injecting negative charge to the residual toner, the time of build-up in negatively charging in the next charging step can be shortened.